Modifier P4 indicates the patient is ASA Physical Status IV — a patient with severe systemic disease that is a constant threat to life. It is assigned by the anesthesia provider and must be supported in the medical record with documentation of ongoing, life‑threatening conditions (for example recent MI, unstable cardiac disease, severe valvular dysfunction, sepsis, ARDS, or end‑stage renal disease not on regular dialysis). P4 is appended only to anesthesia CPT codes (00100–01999) to communicate elevated risk and to support any payer‑specific adjustments.

Billing impact depends on the payer: Medicare (CMS) treats physical status modifiers — including P4 — as informational only and does not increase payment under the Medicare Physician Fee Schedule. Many commercial payers do recognize P4 and add modifying units (commonly 2 units) to the anesthesia payment formula: (Base Units + Time Units + Modifying Units) × Conversion Factor. Because payer policies vary and are evolving (for example, some plans have announced plans to phase out extra reimbursement for physical status modifiers), clinicians should verify each commercial contract’s treatment of P4. Documentation must justify the ASA IV assignment to support coding and any additional reimbursement.